Now more than ever, electric utility power distribution systems are being constructed underground due to public outcry about esthetics of aerial (i.e., above-ground) distribution systems in what is now known as the Not In My Backyard (NIMBY) phenomenon. To appease the NIMBY contingent, power distribution systems formerly constructed of poles, wires, and pole-mounted switches and transformers are being superceded and even replaced by underground systems constructed of conduits or duct-banks, underground vaults, cables, and ground level or sub-ground level switchgear and transformers. Underground systems pose new operational and maintenance challenges by virtue of being largely unseen. In response to these challenges, organizations such as the Institute of Electrical and Electronics Engineers (IEEE) and American National Standards Institute (ANSI) have implemented standards and codes to insure operating personnel safety and proper system performance. However, at times, personnel safety may conflict with system performance. One such standard recommends the grounding (i.e., shielding) of individual underground distribution system components at multiple system points (e.g., cable splices, transformers, switches). By grounding system components (or their enclosures), a system operator seeks to eliminate accessibility to hazardous voltages by operating personnel.
Vacuum interrupting switches are well known for use in power distribution systems for reliable interruption of fault current and load breaking, and have become effective substitutes for air, oil, and SF6 filled switches. When used in underground applications such as vaults or switchgear where there is a high probability of submersion, vacuum interrupting switches are enclosed or encapsulated in electrically insulating material. To ground a submersible vacuum interrupting switch in order to protect personnel from hazardous voltages, the entire switch exterior must be conductive. However, if the switch is grounded, the electric fields inside the device become distorted and reduce the dielectric withstand capability of the open gap during a switch “break” operation. Mitigation of this electric field distortion has so far been elusive to those knowledgeable in the art.
U.S. Pat. No. 4,618,749 to Bohme et al. discloses a vacuum switching apparatus inserted into an insulating material such as epoxy resin. The Bohme et al. switch also has a metallic cover which can be grounded for personnel safety. The disclosed switching apparatus is not integrally molded into the insulating material and a space exists between the apparatus and insulating material. Bohme et al. recognize that the space is susceptible to capacitive discharge due to breakdown of the insulating material (e.g., corona effect) especially during times when the switch contacts are open. Control electrodes embedded in the insulating material attempt to minimize corona effect inside the space by placing voltage stress in the insulating material. It is readily apparent to one knowledgeable in the art that the Bohme et al. device will still suffer from insulating material breakdown. Furthermore, as the switching apparatus is inserted in the preformed insulating housing, the device is expensive and complicated to manufacture.
Thomas & Betts Elastimold® MVI Molded Vacuum Fault Interrupter attempts to overcome the deficiencies of the aforementioned Bohme et al. patent by directly encapsulating the vacuum switch chamber in a molded insulating housing. The voltage stress is now present in the insulating housing which has a much higher breakdown strength. However, since the MVI device is shielded, the presence of a grounded surface in close proximity to the vacuum chamber causes an electric field distortion inside the device which decreases the withstand capability of the open gap. Thus, the device is prevented from operating to its full potential.
The present invention provides a device that overcomes the disadvantages of the prior art. These and other advantages of the invention, as well as additional inventive features, will be apparent from the description of the invention provided herein.